1. Field of the Invention
This invention pertains to isocyanurate foam products and methods for making the same. More specifically, this invention relates to a trimerization reaction of a polymeric isocyanate to yield isocyanurate foams having characteristics of outstandingly high flame resistance and low friability. More specifically, this invention relates to a method for producing a polyisocyanurate foam employing a specific trimerization catalyst at an exceptionally high NCO/OH equivalent ratio.
2. Brief Discussion of the Prior Art
The trimerization reaction of isocyanates to yield isocyanurate linkages has been known for over a 100 years. The isocyanurate linkage is an advantageous polymeric linkage because isocyanurate products exhibit high thermal and hydrolytic stability.
In recent years, the preparation of rigid foams containing isocyanurate linkages has been described by a number of investigators. Burkus, in U.S. Pat. No. 2,993,870, reported that isocyanurate-containing foams could be prepared by trimerizing isocyanate-terminated polyester prepolymers. Nicholas and Gmitter, in Journal of Cellular Plastics, Volume 1 (1965), described a method for making isocyanurate-containing foams in which isocyanate-terminated polyester prepolymer is dissolved in an excess amount of toluene diisocyanate and subsequently trimerized. Foams obtained by either of these two methods have poor flame resistance.
On the other hand, unmodified isocyanurate foams have been found to be highly temperature-resistant. However, such foams are undesirable because they exhibit high friability. Furthermore, when unmodified isocyanurate foams are exposed to fire, they burst into fragments similar to popcorn. This bursting phenomenon was first solved by modifying the isocyanurate product with urethane or other linkages. For example, in French Pat. No. 1,155,768 to Ashida, an isocyanurate foam with lower bursting and lower friability properties was prepared in which the cross-link density was reduced due to the incorporation of urethane or amide linkages. In preparing the isocyanurate polymer disclosed in the French patent, polymeric isocyanate, a polyether, polyester, a carboxyl-terminated polyester or a polymerized fatty acid were reacted in the presence of a trimerization catalyst at an NCO/OH or NCO/COOH equivalent ratio of greater than 2 and an aliphaticity of less than 0.4. Aliphaticity is defined as a weight ratio of active hydrogen containing compound to polymeric isocyanate.
It has been found that higher NCO/OH (or NCO/COOH) equivalent ratios provide greater flame resistance. However, ordinarily high NCO/OH equivalent ratios lead to higher friability. Because of this, urethane-modified isocyanurate foams are generally produced at an NCO/OH equivalent ratio of from about 3.0 to 5.0. Higher ratios result in highly friable foams which cannot be used in any practical applications.
Since unmodified isocyanurate foams are temperature resistant but exhibit extremely high friability and bursting phenomenon when exposed to flame or high temperatures and foams prepared by trimerizing polymeric isocyanate in the presence of a polyether polyol are less friable but exhibit poor flame resistance, a foam which balances between these properties would be desirable. A good balance between low combustion properties and low foam friability would be achieved by an appropriate percent of isocyanate trimer, usually, in the range of about 18 to about 22 percent. Percent trimer levels above this range yield an unacceptably friable foam; while low-trimer contents are linked with increases in the combustibility of the polymer. Heretofore such foams have not been obtainable.
Thus, it would be desirable to provide an isocyanurate foam having high flame resistance, low combustibility and low foam friability.